Update manual_install.md opening for SEO

Search results for download GitLab have https://docs.gitlab.com/omnibus/manual_install.html as the first result instead of about.gitlab.com/install/. Original issue documented in gitlab-com/www-gitlab-com#5095 (closed)

We want people searching for this to land on /install/ and would prefer to see it listed as the first result.

I'd like to include download in the opening as this page links to /install/ and here's my suggestion. Not sure if this matches tone/style guidelines so please update if needed.